Büchner Ring Expansion/Isomerization: Photoactivation of Imide-Annulated N-Heterocyclic Carbenes Affords Vicinal Diamino Cycloheptatrienes

The synthesis and study of an imide-annulated N-heterocyclic carbene are described. The electrophilic nature of the carbene was assessed through analysis of its chalcogen adducts and Ir carbonyl complexes. When subjected to UV irradiation, the carbene underwent cycloaddition with benzene and other arenes to form seven-membered rings with 1,2-disubstitution patterns. The structures of these products were conclusively determined through a series of single-crystal X-ray analyses and other analytical techniques. A mechanism that proceeds through Büchner-type ring expansion followed by isomerization and is facilitated by the unique electronic structure of the carbene is proposed.